1. Field of the Invention
The present invention relates to a self-oscillating DC--DC converter, and in particular, to a self-oscillation DC--DC converter of the on-off type suited for use in power source apparatuses used in industrial equipment and office equipment, such as facsimile apparatuses and printers. Furthermore, the present invention relates to a self-oscillating DC--DC converter that can have an improved conversion efficiency, and can operate stably even when there is no load or when there is a light load.
2. Description of the Related Art
The ON-OFF types of DC--DC converters can be of a separate excitation oscillation type having separate excitation and oscillation, or of a self-oscillation type. The separate excitation oscillation type operates by means of an external oscillator, while the self-oscillation type performs a switching operation, and does not have a special oscillator. In general, the separate excitation oscillation type uses more parts when compared to the self-oscillation type; the resultant increase in cost means that the separate excitation type is rarely used.
A typical example of the self-oscillation type is the RCC (Ringing Choke Converter).
FIG. 1 is a block diagram showing the constitution of the main portions of a conventional type switching regulator using the RCC method. The conventional switching regulator is provided from a primary side circuit on the input side and a secondary side circuit on the output side. The primary side circuit comprises a diode D2, a transistor Q1, and a resistor R1, while the secondary side circuit comprises a diode D and a capacitor C. A transformer T1 is provided between the primary side circuit and the secondary side circuit.
The switching regulator shown in FIG. 1 has the transistor Q1, as a switching element, repeatedly perform an ON-OFF oscillation with a predetermined period, so that a square wave is generated on the second side (output side) of the transformer T1. This wave is converted into a DC voltage. The switching regulator stores the energy in the inductance (L) of the primary winding of the transformer T1 during the period when the transistor Q1 is in the ON state; when the transistor Q1 is in the OFF state, this energy is supplied to the smoothing capacitor C of the secondary side circuit and the load (not shown in the figure) via the rectifier diode D of the secondary side circuit.
FIG. 2 is a timing chart for describing the operation of the switching regulator of the RCC method of FIG. 1. The numbers of the signal wave-form of FIGS. 2A, 2B and 2C correspond to each of the label positions in FIG. 1.
The transistor Q1 has a collector voltage as shown in FIG. 2A, and generates a current Ic of a sawtooth wave as shown in FIG. 2B. At this time, the main portion of the switching loss is generated when the transistor Q1 is in the OFF state.
In order to reduce the switching loss when the transistor Q1 is in the OFF state, a diode D2 is inserted at a position as shown in FIG. 1. However, as shown in FIG. 2C, the switching loss increases the time when the transistor Q1 is in the ON state. As a result, there is hardly any reduction in the overall switching loss.
FIG. 3 is a view showing a circuit configuration of the main portions of a conventional switching regulator of the RCC type. The primary side circuit of this switching regulator comprises a transistor Q1, photocouplers PC2, resistance R1 and a capacitor C1. The secondary side circuit of this switching regulator comprises a diode D1, a zener diode ZD, an amplifier Amp1 and capacitor C2, and resistors R2-R5. In FIG. 3, (S), (B) and (P) are each of the windings of the transformer T1. In addition, the current I1 and the current I2, shown by arrows represent the current of the primary side circuit, and the current of the secondary side circuit respectively.
FIG. 3 shows a detailed circuit configuration of the switching regulator of FIG. 1, thus the basic operation is the same as that of FIG. 1. When the transistor Q1 turns ON, the curent I1 increases, and when the transistor Q1 turns OFF, the capacitor C1 is charged by the current I1. Furthermore, energy is stored in the transformer T1. When the energy, stored in the transformer T1 increases beyond a predetermined value, energy is discharged to the secondary side circuit and an output voltage is generated.
Accordingly, as described above with respect to FIG. 2, a conventional switching regulator of the RCC type has a poor switching conversion efficiency because the switching loss becomes large irrespective of whether or not the diode D2 is provided. Additionally, because it is a self-oscillation switching regulator, when there is only a light load applied to the circuit, and the frequency becomes higher, unstable operation, such as intermittent operation, easily occurs.